Tuning the charge distribution of Co–N–C active sites for enhanced trifunctional electrocatalysis

Abstract

Previous studies have often focused on optimizing electrocatalytic performance by altering the type of metal centers or the morphological structure of catalysts to modulate their electronic structures and active sites. However, the precise regulation of metal valence states to enhance catalytic performance—particularly for trifunctional electrocatalysis in the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER)—remains in the exploratory stage. In this study, crystalline Co–N₆ materials with controllable valence states were carefully selected as precursors. Through carbonization, Co–N–C materials with similar carbon frameworks but distinct metal valence states (Co²⁺ and Co³⁺) were successfully prepared. Both experimental tests and theoretical calculations demonstrate the superior trifunctional electrocatalytic performance of Co²⁺–N–C. Density functional theory (DFT) calculations provide evidence for the more favorable intermediate adsorption energies and lower reaction energy barriers exhibited by Co²⁺–N–C. This performance advantage stems from the 3d⁷ electronic configuration of Co²⁺, which optimizes electron cloud density and strengthens Co–N bonding interactions. This work presents a new strategy for designing highly efficient electrocatalysts by elucidating the regulatory role of metal valence states.

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Introduction

Driven by the continuous development of energy conversion and storage technologies, electrocatalytic reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), have become the core driving forces in many fields, such as fuel cells, metal–air batteries, and water electrolysis.^1–6 In the process of these reactions, materials with high electrochemical activity and durability are undoubtedly the key factors for achieving efficient reactions. Noble metal catalysts, such as Pt-based nanomaterials, exhibit excellent bifunctional catalytic properties in the HER and ORR, while Ru-based and Ir-based catalysts perform well in the OER. However, their rarity and high cost have greatly limited their application prospects in large-scale production.^7–16 Therefore, optimizing non-noble inorganic material catalysts to achieve efficient energy conversion is not only a core step in promoting the wide application of new energy storage and conversion technologies but also a significant research challenge currently faced in this field.^17–19

Currently, researchers' exploration in the field of multifunctional catalysts mainly focuses on two key approaches. On the one hand, they are fully committed to the meticulous design and precise synthesis of highly efficient catalysts with catalytically active centers. On the other hand, they are conducting in-depth analyses of the reaction mechanisms and processes of catalysts, aiming to enhance the comprehensive understanding of catalysts and broaden their application scope.^20–23 Numerous research groups have achieved a series of highly inspiring results in related studies. For example, the research group led by Hao Ming Chen conducted an in-depth investigation into the influence of metal valence states on electrocatalysis in metal oxide systems. They successfully confirmed that CoOOH formed from Co²⁺ serves as the oxidation site for water electrolysis. This discovery highlights the significant role of metal valence states in the formation of key electrocatalytic reaction sites in specific materials.²⁴ The research group led by Yaping Li took an innovative approach. They ingeniously incorporated Co^2+/3+ sites into the NiFe-LDH structure and conducted an in-depth investigation into its effect on the OER activity. By simplifying the structure, they demonstrated the efficacy of valence states. The results showed that the performance was significantly improved after Co³⁺ doping.²⁵ Hu Xile's group elucidated the influence of valence state modulation on the electrocatalytic reduction of CO₂ in Fe³⁺–N–C catalysts, providing a clear rationale for the development of highly efficient electrocatalysts through valence state engineering.²⁶

Previous studies indicate that Co–N–C catalysts possess notable structural advantages:^27–29 (1) at the atomic level, by meticulously designing the precursor Co–N₆—which enables effective modulation of the metal center properties—allows for the construction of Co–N–C materials with precisely controlled cobalt valence states (Co²⁺ or Co³⁺) through subsequent transformation. Additionally, the carbon substrate itself can provide additional adsorption sites, and the two (i.e., the Co–N active sites derived from Co–N₆ transformation and the adsorption sites on the carbon substrate) synergistically increase the number of active sites. (2) From a structural perspective, Co–N–C materials exhibit a two-dimensional planar architecture. Its high specific surface area not only facilitates the rapid access of electrocatalytic reaction substrates (e.g., protons in HER, and O₂ in OER and ORR) to the catalytic sites—thereby effectively increasing the probability of reaction contact³⁰—but also shortens the transport pathways for ions and electrons within the material, reduces energy loss, and thus accelerates the kinetic process of electrocatalytic reactions.^31,32 Based on these advantageous properties, this work meticulously selected precursors and successfully synthesized crystalline MOFs featuring Co–N₆ structures with well-defined metal valence states (Fig. 1). Subsequent carbonization under a N₂ atmosphere—effectively suppressing abnormal valence transitions of Co species during pyrolysis and ensuring valence stability—facilitated the directed transformation of the Co–N₆ motifs into Co–N–C materials. This process yielded two target catalysts: Co²⁺–N–C and Co³⁺–N–C. Building upon this foundation, we further investigated the influence of charge regulation on the trifunctional electrocatalytic performance of Co–N–C catalysts for ORR, OER, and HER. Both experimental measurements and theoretical calculations consistently demonstrated that Co²⁺–N–C exhibits superior catalytic activity and stability across all three reactions. This marked performance disparity clearly establishes the metal center's valence state as a critical factor governing the electrocatalytic efficacy of Co–N–C materials. These findings not only provide a new perspective for understanding the structure–activity relationships of MOF-derived electrocatalysts, but also open a promising avenue for optimizing multifunctional electrocatalytic performance through deliberate valence state engineering.

Fig. 1 Schematic diagram of the synthesis of Co²⁺–N–C and Co³⁺–N–C from Co²⁺-MOF-1 and Co³⁺-MOF-2, respectively (with metal valence centers of Co²⁺ and Co³⁺).

Experimental section

Materials

All analytical-grade chemicals and used as received. CoCl₂·6H₂O, Co(CH₃COO)₂·4H₂O, ethyl 1H-tetrazole-5-carboxylate (ETC), N,N-bis[1(2)H-tetrazol-5-yl]amine (H₃bta), sodium azide, sodium dicyanamide, hydrochloric acid, absolute ethanol, and isopropanol were obtained from Sinopharm Chemical Reagent Co., Ltd and Shanghai Aladdin Biochemical Technology Co., Ltd. Nafion PFSA polymer (5%) was supplied by Chemours. Distilled water (with a purity exceeding 18 MΩ cm), was purified through a microporous membrane system.

Catalyst synthesis

[Co(HCN₄)₂·(H₂O)_2/9]·1.4H₂O (denoted as Co²⁺-MOF-1)³³. Co(CH₃COO)₂·4H₂O (49.8 mg, 0.2 mmol) was mixed with ETC (56.8 mg, 0.4 mmol) in 15 mL of distilled water. The mixture was subsequently placed into a polytetrafluoroethylene (PTFE) container and maintained at 125 °C for a duration of 20 h. The product was filtered, rinsed multiple times with ethanol, and vacuum-dried at 50 °C for 12 h. The precursor Co²⁺-MOF-1 was thus prepared with a yield of approximately 70%.

Co(HBTA)₃·4.5H₂O (denoted as Co³⁺-MOF-2)³⁴. CoCl₂·6H₂O (23.8 mg, 0.1 mmol) and H₃bta (45.9 mg, 0.3 mmol) were dissolved and combined in 50 mL of distilled water. The mixture was subsequently placed into a PTFE container and maintained at 125 °C for a duration of 20 h. The resulting clear mixed solution was filtered, and the filtrate was placed in a sealed environment at 6 °C for slow crystallization. Pink crystals precipitated out. After being washed with ethanol and dried, the precursor Co³⁺-MOF-2 was finally prepared with a yield of approximately 34%.

Carbonization of precursors³⁵. A 200 mg sample of Co²⁺-MOF-1 precursor was loaded into a tube furnace within a magnetic boat for calcination. The precursor was heated to 700 °C (5 °C min⁻¹) under a N₂ atmosphere and held at that temperature for 2 h. Upon reaching room temperature naturally, the product was retrieved, stirred in 1 M HCl overnight, washed with distilled water until neutrality (pH ≈ 7), and dried under vacuum at 50 °C for 24 h. The resulting sample was named Co²⁺–N–C. Prepare Co³⁺–N–C following the same steps, except that the precursor is replaced with Co³⁺-MOF-2.

Results and discussion

Characterization of structural properties of Co²⁺–N–C and Co³⁺–N–C

First, the sample catalysts were characterized by powder X-ray diffraction (PXRD) to analyze their crystal phases and structures. From the PXRD pattern of the synthesized precursor (details see Fig. S1), it can be found that the diffraction peaks of Co²⁺-MOF-1 correspond one-to-one to the peak positions of the simulated pattern of Co²⁺-MOF-simulated,³³ indicating that it conforms to the structure of [Co(HCN₄)₂·(H₂O)_2/9]·1.4H₂O. In contrast, the diffraction peaks of Co³⁺-MOF-2 can be well-matched with those of Co³⁺-MOF-simulated (CCDC 661223), which means that it meets the structural characteristics of Co(HBTA)₃·4.5(H₂O).³⁴ Additionally, the thermogravimetric analysis combined with differential scanning calorimetry (TG-DSC) results (as shown in the supplementary Fig. S2) indicate that the synthesized precursor Co²⁺-MOF-1 stabilizes in mass and ceases to change after the temperature rises to 620 °C, while Co³⁺-MOF-2 reaches a constant mass after attaining 550 °C. Taking into account the heat resistance of the materials as well as the structural characteristics of the target product, 700 °C was selected as the carbonization temperature. Under these temperature conditions, the precursors Co²⁺-MOF-1 and Co³⁺-MOF-2 with different valence states were subjected to carbonization treatment under nitrogen protection. Benefiting from the inert nature of nitrogen, the organic ligands of the precursors were fully decomposed during the carbonization process, allowing the metal ions to effectively react with the nitrogen source. Consequently, Co–N–C materials with specific structures and performance characteristics were successfully constructed (Fig. 1). These materials were able to stably maintain the original valence states of their precursors.^35,36 The PXRD patterns of Co²⁺–N–C and Co³⁺–N–C catalysts (Fig. 2a) revealed prominent diffraction peaks at 2θ ≈ 24.3° matched the (002) plane of graphitic carbon, indicating the presence of graphitized structures in the synthesized material.^37,38 This feature implies that the material exhibits a graphite-like layered structure at the microscopic level, which, with its unique electron delocalization properties and well-ordered layered structure, usually contributes positively to the enhancement of the material's electrical conductivity and other related properties.³⁹

Fig. 2 Co^2+/3+–N–C catalysts: (a) PXRD comparison spectra; (b) Raman comparison spectra; (c) IR spectra of Co-MOF precursors with different metal valence centers; (d) IR spectra of Co–N–C catalysts with different metal valences.

Raman spectroscopy was employed to conduct a more in-depth analysis of the graphitization degree and defect features of the two Co–N–C catalysts. As shown in Fig. 2b, both Co²⁺–N–C and Co³⁺–N–C materials with distinct metal valence states display prominent peaks at 1340 cm⁻¹ and 1588 cm⁻¹, representing the D-band and G-band of carbon materials, respectively.⁴⁰ Specifically, the D-peak represents the characteristic vibration mode of sp³ defect sites in carbon atoms, while the G-peak reflects the vibration of sp²-bonded atoms. Notably, the intensity ratio I_D/I_G of these two peaks can serve as a key indicator of the degree of graphitization as well as the defect density of the material.⁴¹ Through the integral fitting process of the spectra, the I_D/I_G values of Co²⁺–N–C and Co³⁺–N–C were found to be 1.21 and 1.32, respectively. Generally, a lower I_D/I_G value indicates that the corresponding material has a better graphitization level, which is more beneficial for the efficient transmission of electrons.⁴² This conclusion is also consistent with the intensity of the relative carbon peak (002) lattice plane shown in the PXRD data. The structural characteristics of the material before and after carbonization were characterized and analyzed by Fourier transform infrared (FT-IR) technology. In Fig. 2c, the FT-IR spectra of the precursors Co²⁺-MOF-1 and Co³⁺-MOF-2 are presented. The two curves have their own characteristic peaks at different wavenumbers. When we focus on the FT-IR spectra of carbonized Co²⁺–N–C and Co³⁺–N–C in Fig. 2d, the spectral profiles of both materials exhibit similar peak positions, peak shapes, and relative intensities at several key wavenumber positions over a range of wavenumbers from 4000 cm⁻¹ to 400 cm⁻¹. During the carbonization process, this fully indicates that the Co^2+/3+–N–C materials generated from the two precursors have significant similarities in chemical structure, which provides a solid foundation for the subsequent in-depth investigation of the structure–activity relationship between their properties and structures.

The morphological and microstructural features of the two catalysts were systematically characterized using scanning electron microscope (SEM) and transmission electron microscopy (TEM). SEM results reveal that Co²⁺–N–C exhibits a uniformly distributed cubic morphology (Fig. 3a–c) with an average particle size of approximately 100 nm. The abundant interstitial voids between particles are conducive to enhanced mass transport during electrocatalytic reactions.⁴³ In contrast, Co³⁺–N–C predominantly displays an irregular rod-like morphology (Fig. 3d–f) with significantly larger particle dimensions, averaging around 500 nm. TEM images in Fig. S3a and e reveal that Co²⁺–N–C exhibits a more homogeneous structure with smaller particle sizes and an ordered arrangement, in contrast, Co³⁺–N–C has significantly larger particle sizes with a relatively loose distribution. Corresponding energy-dispersive spectroscopy (EDS) elemental mapping (Fig. S3b–d and f–h) confirms the successful doping and uniform distribution of Co, N, and C in both materials. Notably, nitrogen in Co²⁺–N–C demonstrates a higher distribution uniformity, suggesting a more regular arrangement of active sites that contributes to its superior electrocatalytic performance.

Fig. 3 (a–c) SEM images of Co²⁺–N–C at different sizes; (d–f) SEM images of Co³⁺–N–C at different sizes.

Furthermore, X-ray photoelectron spectroscopy (XPS) will be used to further analyze the elemental composition and coordination environment of the Co–N–C catalyst. Evidently, as demonstrated by the full spectrum presented in Fig. 4a, the characteristic signals corresponding to Co, O, N, and C can be significantly captured. Specifically, the Co 2p XPS spectrum of the Co²⁺–N–C material (Fig. 4b) shows two main peaks at approximately 780.36 eV and 796.10 eV, which are assigned to Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, respectively. Their binding energy positions agree well with those reported in ref. 44 The presence of distinct satellite peaks (Sat.) adjacent to these main peaks further confirms the existence of Co²⁺ species. Meanwhile, in Co³⁺–N–C materials, the signals located around 779.15 eV and 794.51 eV should be attributed to the Co³⁺ 2p_3/2 and Co³⁺ 2p_1/2 energy levels.⁴⁵ In the Co–N–C material system, the unique coordination mode between nitrogen and cobalt atoms exerts a critical influence on the electronic structure of cobalt. Compared to Co³⁺ (with a 3d⁶ configuration), Co²⁺ possesses a 3d⁷ configuration, resulting in a higher number of d electrons. The higher initial d-electron density enhances the interaction between the Co²⁺ center and the electronegative N/C ligands, promoting greater delocalization of electron density towards the ligands. This intensified metal–ligand charge transfer effect consequently leads to a relatively weakened effective electron shielding at the Co²⁺ site under the final state probed by XPS, thereby increasing its core-electron binding energy.^46–49 This manifests as a shift of the Co 2p peaks for Co²⁺–N–C to higher binding energies compared to those for Co³⁺–N–C. Fig. 4c presents the C 1s spectra of the Co²⁺–N–C and Co³⁺–N–C catalysts. Distinct peaks corresponding to C–N and C=N bonds can be clearly observed, which strongly confirms the existence of nitrogen doping within the carbon matrix.⁵⁰ The relative proportions of different carbon species further reveal the structural characteristics, as summarized in Table S1. After the peak-fitting treatment of the N 1s signals shown in Fig. 4d, four characteristic peaks of nitrogen types located at 398.09 eV, 400.17 eV, 401.66 eV, and 403.81 eV are clearly presented. These peaks are attributed to pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and oxidized nitrogen in sequence.⁵¹ According to previous studies, within the catalyst system, the metal–nitrogen bonds constructed by pyridinic nitrogen have been widely recognized as highly efficient active sites in the ORR and OER processes.^52,53 Focusing on the Co–N–C samples we prepared (for specific relevant integral proportion data, see SI Table S2), the atomic content of pyridinic nitrogen in the Co²⁺–N–C catalyst is 54.42%, and this catalyst contains the pyridinic nitrogen–Co structural unit. From the perspective of the coordination environment, the Co²⁺ ion has a 3d⁷ electronic structure, endowing it with certain electron-donating and electron-accepting capabilities. During the construction of the Co²⁺–N–C system, the electron cloud density distribution around nitrogen atoms is mutually adapted to the electronic structure of Co²⁺ ions, creating a coordination environment conducive to the formation of metal–nitrogen bonds. This apt environment promotes the interaction of Co²⁺ with nitrogen atoms and increases the number of metal–nitrogen bonds, which promises excellent electrocatalytic activity for Co²⁺–N–C catalysts.⁵⁴

Fig. 4 Co^2+/3+–N–C catalysts: (a) XPS full spectrum; (b) high-resolution Co 2p spectrum; (c) high-resolution C 1s spectrum; (d) high-resolution N 1s spectrum.

Electrochemical performance analysis

ORR electrocatalytic activity. First, by means of the rotating disk electrode technique, the two synthesized catalysts, Co²⁺–N–C and Co³⁺–N–C, were respectively placed in an oxygen-saturated and a nitrogen-saturated 0.1 M KOH electrolyte environment. Cyclic voltammetry (CV) was used to conduct tests to evaluate the ORR performance of the two catalysts. The test results indicate that, in the oxygen-saturated electrolyte system, a significant cathodic peak can be clearly observed (see SI Fig. S4 for details). This phenomenon confirms that the two samples possess ORR activity.⁵⁵ Notably, the Co²⁺–N–C oxygen electroreduction peak was more positive (0.77 V vs. RHE), indicating a superior electrocatalytic ORR activity. Subsequently, as shown in Fig. 5a, linear sweep voltammetry (LSV) curves were obtained under the conditions of a rotation rate of 1600 rpm and a scan rate of 5 mV s⁻¹. Among the tested catalysts, compared with Co³⁺–N–C, Co²⁺–N–C exhibits a higher onset potential (E_onset = 0.86 V vs. RHE) and half-wave potential (E_1/2 = 0.77 V vs. RHE), and its limiting current density is relatively large (3.6 mA cm⁻¹). Evidently, the ORR catalytic activity of Co²⁺–N–C is superior to that of the Co³⁺–N–C catalyst. Although Co²⁺–N–C is slightly inferior to commercial Pt/C (E_1/2 = 0.83 V), when comparing its onset potential and half-wave potential with those of the Co–N–C ORR catalysts reported in recent literature (specific data are shown in Table S3), it is found that the catalytic activity of the Co²⁺–N–C catalyst is comparable to that of most state-of-the-art transition-metal-based catalysts. To reveal the ORR kinetics, the Tafel slopes and kinetic current densities (J_k) of Co²⁺–N–C and Co³⁺–N–C were calculated (eqn (S3)). As can be seen from Fig. 5d, the Tafel slope value of Co²⁺–N–C is 107.5 mV dec⁻¹, which is significantly lower than that of Co³⁺–N–C (125.8 mV dec⁻¹). This indicates that the Co²⁺–N–C catalyst has a faster reaction kinetics during the ORR process, which is undoubtedly more favorable for the efficient promotion of the electrochemical reaction.⁵⁶ At the same time, we also replaced the electrolyte solution with 0.1 M HClO₄ and 0.1 M K₂SO₄ for the test. From the variation of its measured LSV curve (Fig. 5b and c) and the corresponding Tafel slope (Fig. 5e and f), Co²⁺–N–C also showed the same direction of the difference in performance, and still maintained the performance advantage over Co³⁺–N–C. The kinetic current density was calculated according to the K–L equation (eqn (S4) and (S5)).⁵⁷ Under the test condition of 0.80 V, the kinetic current density J_k value of Co²⁺–N–C was 3.21 mA cm⁻², which was larger than that of Co³⁺–N–C (3.06 mA cm⁻²). Generally speaking, in electrochemical reactions, the higher the onset potential and half-wave potential are, the larger the kinetic current density is, and the faster the kinetic rate of the corresponding reaction is.⁵⁸ To deeply investigate the ORR activities of Co²⁺–N–C and Co³⁺–N–C, this study systematically measured the LSV curves at rotation speeds ranging from 400 to 2025 rpm. The corresponding data are shown in Fig. 5g and S6. Based on the LSV curves, the number of electrons transferred during the ORR process was precisely calculated using eqn (S6) and (S7). The results demonstrate that at a potential of 0.4 V (vs. RHE), the electron transfer numbers for Co²⁺–N–C and Co³⁺–N–C are 3.32 and 3.08, respectively. This indicates that Co²⁺–N–C catalyzes the oxygen reduction reaction more closely to the 4e⁻ transfer pathway,⁵⁹ implying that Co²⁺–N–C catalysts are more inclined to the efficient reaction pathway for the direct reduction of oxygen to water, which in turn improves the energy conversion efficiency of the whole ORR process.

Fig. 5 ORR performance diagrams of two Co–N–C catalysts in different electrolytes: (a) LSV curves in an alkaline electrolyte (0.1 M KOH); (b) LSV curves in an acidic electrolyte (0.1 M HClO₄); (c) LSV curves in a neutral electrolyte (0.1 M K₂SO₄); (d) Tafel slopes in a 0.1 M KOH electrolyte; (e) Tafel slopes in a 0.1 M HClO₄ electrolyte; (f) Tafel slopes in a 0.1 M K₂SO₄ electrolyte; (g) LSV curves and K–L illustrations of Co²⁺–N–C at 400–2025 rpm in a 0.1 M KOH electrolyte; (h) EIS diagrams in a 0.1 M KOH electrolyte; (i) i–t graphs in a 0.1 M KOH electrolyte.

In this work, cyclic voltammetry (CV) was applied to measure the current response in the non-faradaic potential region (Fig. S7). The electrochemical double-layer capacitance (C_dl) was determined by further calculating the slope of the linear fit so as to visually assess the actual electrochemically active area (ECSA) of the catalysts in the catalytic process.⁶⁰ As can be seen from Fig. S5, the C_dl value of Co²⁺–N–C (3.32 mF cm⁻²) is higher than that of Co³⁺–N–C (2.38 mF cm⁻²). Given that the slope of the C_dl is directly proportional to the actual ECSA, it means that under the same conditions, Co²⁺–N–C has a larger electrochemically active surface area and exposes relatively more active sites on its surface. These active sites are regulated by the metal valence state centers. Therefore, compared with Co³⁺–N–C, the metal valence state centers of Co²⁺–N–C are more favorable for the catalysis of the ORR, thus exhibiting higher catalytic activity. Fig. 5h shows the results of the electrochemical impedance spectroscopy (EIS) tests for the two catalysts. Obviously, the Nyquist plot of Co²⁺–N–C exhibits a narrower semi-circular arc, which intuitively indicates that it has a lower resistivity.⁶¹ Judging from the specific data, the resistance value of Co²⁺–N–C is 2.3 Ω, which is lower than the 4.6 Ω of Co³⁺–N–C. The low-resistance characteristic enables Co²⁺–N–C to conduct electrons rapidly, thereby effectively improving the electrocatalytic efficiency and demonstrating significant advantages in terms of kinetics. Stability is a key indicator of catalyst performance, for which the electrochemical stability of two Co–N–C materials was further tested. As can be seen from Fig. 5i, under the constant-voltage test in a 0.1 M KOH electrolyte solution, both of them performed well. Co²⁺–N–C could still maintain approximately 91.3% of the initial current after 252 hours, and Co²⁺–N–C could also maintain about 90.3% of the initial current after the same period. This indicates that this type of Co–N–C material has good electrocatalytic stability in a harsh electrolyte environment.

OER electrocatalytic activity. In the exploration of the ORR performance of Co²⁺–N–C and Co³⁺–N–C materials, it has been found that the Co²⁺–N–C material exhibits better performance. Considering the relevance of oxygen electrode reactions and the potential influence of the materials' own characteristics on different reactions, next, we will focus on the research of the OER performance of these two materials. To accurately evaluate the OER catalytic performance of the synthesized Co–N–C materials, 0.1 M and 1.0 M KOH electrolyte solutions were selected to set up a three-electrode system: the working electrodes were modified with the prepared Co²⁺–N–C and Co³⁺–N–C materials, while the reference electrode was an RHE and the counter electrode was a carbon rod. By measuring the current density at different potentials, polarization curves were plotted. Then, key parameters such as the OER onset potential, overpotential, and Tafel slope of the materials were obtained to conduct an in-depth exploration of their catalytic activity and kinetic characteristics under different electrolyte concentrations.

According to the LSV curves in Fig. 6a and d, in the two electrolyte solution environments shown, it can be clearly seen that the catalytic performance of the two Co–N–C catalysts is indeed slightly inferior to that of the commercial IrO₂ catalyst. However, it is worth noting that among these two Co–N–C catalysts, the OER catalytic efficiency of Co²⁺–N–C is always superior to that of Co³⁺–N–C. Judging from the trend of the curves, at each potential, the current density of Co²⁺–N–C is greater than that of Co³⁺–N–C. This indicates that Co²⁺–N–C can more effectively promote electron transfer and the progress of the reaction during the OER process, thus having relatively higher catalytic activity.⁶² This performance difference may stem from the different electronic structures and coordination environments of Co²⁺ and Co³⁺, which result in different interactions between them and the N–C support, as well as different adsorption and desorption abilities for OER reaction intermediates. Fig. 6b and e shows that regardless of the concentration, the overpotential of Co²⁺–N–C is always lower than that of Co³⁺–N–C. This indicates that the Co²⁺–N–C catalyst needs to overcome a relatively smaller energy barrier during the electrochemical reaction, suggesting that its catalytic performance is relatively better and the reaction can proceed more easily. From the perspective of OER kinetics, in a 0.1 M KOH electrolyte solution environment, the Tafel slopes of Co²⁺–N–C and Co³⁺–N–C were calculated (as shown in Fig. S8a). Among them, the Tafel value of Co²⁺–N–C is 130.5 mV dec⁻¹, which is significantly lower than that of Co³⁺–N–C (139.0 mV dec⁻¹). This result strongly indicates that the Co²⁺–N–C catalyst exhibits faster reaction kinetics during the corresponding reaction process, which is more favorable for the rapid progress of the electrochemical reaction. It is worth mentioning that the same conclusion about the Tafel slope was obtained in the 1.0 M KOH electrolyte system (as shown in Fig. S8b).⁶³

Fig. 6 OER characterization of Co–N–C with different valence states and commercial IrO₂ in 0.1 M and 1.0 M KOH: (a–c) 0.1 M KOH: LSV curves, overpotential at 10 mA cm⁻², and Co oxidation CV curves; (d–f) 1.0 M KOH: LSV curves, overpotential at 10 mA cm⁻², and Co oxidation CV curves.

As can be clearly seen from the CV curves shown in Fig. S9, in the two electrolyte solution systems of 0.1 M KOH and 1.0 M KOH, the redox peak positions of the two Co–N–C materials fall within the ranges of 0.916–1.216 V and 0.926–1.226 V, respectively. A more in-depth analysis of the CV curves within these two narrower potential intervals (as shown in Fig. 6c and f) shows that the Co–N–C materials centered on different metal valence states exhibit different electrochemical active areas. According to the attached eqn (S8), the integral operation was performed for the charges in the intervals corresponding to the highest voltage peaks (i.e., the oxidation peaks, located near 1.16 V and 1.108 V, respectively) (see Table S5 for details). The calculations showed that the electrochemically active surface area of Co²⁺–N–C significantly surpassed that of Co³⁺–N–C. This result clearly indicates that in the catalytic process of the OER, the catalytic sites where Co²⁺ is located have better activity, which in turn enables Co²⁺–N–C to exhibit better performance in this catalytic process.⁶⁴ In addition, it can also be clearly observed from the figure that within different potential ranges, the changing trends of the current densities of the two types of materials also differ. This is most likely due to the differences in their electronic structures and surface properties, which further reveals the crucial influence of the metal valence states on the catalytic performance of the materials. According to Fig. S5 and S10, in the two different electrolyte solution systems, the C_dl value of Co²⁺–N–C is significantly larger, which further verifies that it has a larger electrochemically active area.

HER electrocatalytic activity. To investigate the HER catalytic performance of Co–N–C materials with different metal valence states at the center, a three-electrode system was used at room temperature. The tests were conducted using 0.5 M H₂SO₄ (acidic), 0.1 M K₂SO₄ (neutral), and 0.1 M KOH (alkaline) as electrolytes, respectively. Using 20 wt% commercial Pt/C as the benchmark, all potentials were referenced to the RHE, with a scan rate of 5 mV s⁻¹. From the LSV curve test results in Fig. 7a–c, it can be seen that under different electrolyte solution conditions such as acidic, neutral, and alkaline conditions, the catalytic performance of Co²⁺–N–C is superior to that of Co³⁺–N–C. Meanwhile, according to the overpotential comparison chart in Fig. S11, in the acidic medium, when the current density reaches 10 mA cm⁻², the corresponding overpotential of Co²⁺–N–C is 412 mV. To more comprehensively analyze the HER kinetic characteristics, we performed a fitting operation on the Tafel plots according to the attached eqn (S3). The obtained results are shown in Fig. 7d and S12. Taking the 0.5 M H₂SO₄ electrolyte solution as an example, the slope of Co²⁺–N–C was determined to be 170.9 mV dec⁻¹ through calculation. This value is significantly larger compared with that of the commercial 20% Pt/C (35.01 mV dec⁻¹), but slightly smaller than that of Co³⁺–N–C (178.5 mV dec⁻¹). Nonetheless, Co²⁺–N–C exhibits superior electrocatalytic performance to Co³⁺–N–C. This phenomenon strongly implies that the catalytic active centers of Co²⁺–N–C significantly promote the Volmer–Heyrovsky kinetic mechanism.⁶⁵ The results of the C_dl values shown in Fig. 7e and S5, S7, S13, S14 collectively indicate that Co²⁺–N–C possesses more electrocatalytic active sites and thus exhibits superior HER electrocatalytic performance. To thoroughly investigate the stability of the two synthesized Co–N–C catalysts under acidic conditions, catalytic reaction tests were conducted in a 0.5 M H₂SO₄ electrolyte solution respectively. The comparison of the corresponding curves before and after the reaction is shown in Fig. 7f. From this figure, it can be seen that the LSV curves of the two Co–N–C materials before and after catalysis basically overlap. That is, there is almost no change in their potentials, which further confirms that the materials possess more stable electrochemical performance in acidic electrolyte solutions.

Fig. 7 HER performance curves of Co–N–C materials with two metal valence states and commercial 20% Pt/C materials in different electrolytes: (a) LSV curves in an acidic electrolyte (0.5 M H₂SO₄); (b) LSV curves in a neutral electrolyte (0.1 M K₂SO₄); (c) LSV curves in an alkaline electrolyte (0.1 M KOH); (d) Tafel slopes in a 0.5 M H₂SO₄ electrolyte; (e) double-layer capacitances (C_dl) in a 0.5 M H₂SO₄ electrolyte; (f) comparison diagrams of LSV curves before and after catalysis in a 0.5 M H₂SO₄ electrolyte.

Density functional theory (DFT) calculations

Although valence state modulation is a critical strategy for optimizing the electronic structure and catalytic performance of M–N–C catalysts, previous studies have primarily focused on Fe-based systems. For instance, research on the CO₂ reduction reaction (CO₂RR) has revealed intrinsic activity differences between Fe³⁺ and Fe²⁺ sites (e.g., pyrrolic Fe^IIIN₄ vs. pyridinic Fe^IIN₄).^26,66 Inspired by these findings, and to elucidate the influence of cobalt valence states on the selectivity across various electrocatalytic reactions, we adopted the structural classification from ref. 67 Using pyridinic Co²⁺–N₄ and pyrrolic Co³⁺–N₄ as computational models (Fig. 8a and b), DFT calculations were performed to probe the energy profiles of the reactions. The results demonstrate that the Co²⁺ sites simultaneously optimize the adsorption free energies of multiple key intermediates (O*, OH*, OOH* and H*) involved in ORR, OER, and HER, thereby significantly lowering the overpotentials of each reaction and endowing the material with superior trifunctional electrocatalytic activity. Carbon vacancies were introduced to maintain charge neutrality and structural stability of the Co³⁺ coordination environment, rather than to intentionally create additional active sites. To examine whether carbon vacancies could serve as potential reaction centers, we explicitly evaluated O₂ adsorption at the carbon vacancy sites. Further detailed descriptions can be found in Fig. S15 and S16 of the SI.

Fig. 8 (a) ORR reaction configurations on Co sites in Co²⁺–N–C. (b) ORR reaction configurations on Co sites in Co³⁺–N–C. (c–e) Free energy diagrams for ORR, OER, and HER pathways on Co^2+/3+–N–C catalysts.

The Co²⁺ valence state endows the active centers (Co–N–C) with a distinct electronic configuration. Compared to higher-valence Co³⁺, Co²⁺ possesses more d-electrons, resulting in a density of states near the Fermi level that is more favorable for forming neither too strong nor too weak “moderate adsorption” with key reaction intermediates (O*, OH*, OOH* and H*).⁶⁸ For both ORR and OER, the catalyst simultaneously reduces the Gibbs free energy of the key intermediates O*, OH* and OOH* (Fig. 8c and d), preventing any single step from becoming an insurmountable energy barrier and thereby facilitating a more efficient four-electron transfer pathway. Specifically, during the ORR process, the Gibbs free energy change (ΔG) for Co²⁺–N–C is (−0.56 eV), which is lower than that of Co³⁺–N–C (−0.40 eV). Similarly, for the OER process, the ΔG value of Co²⁺–N–C is 1.72 eV, also lower than that of Co³⁺–N–C (1.88 eV). For the HER, the hydrogen adsorption free energy (ΔG_H*) serves as a descriptor for the rate-determining step, with an ideal value approaching 0 eV. The ΔG_H* value of Co²⁺–N–C (−0.04 eV) lies nearly perfectly at the apex of the volcano plot (Fig. 8e), indicating that the adsorption/desorption process of H* approaches thermodynamic equilibrium with a minimal kinetic barrier. This performance is markedly superior to that of Co³⁺–N–C (−0.10 eV), and thus exhibits weaker H* adsorption. We have performed additional electronic structure analyses, including partial density of states (PDOS) calculations, to further elucidate the origin of the different adsorption behaviors between Co²⁺ and Co³⁺ sites (detailed description can be found in Fig. S17 of the SI).

Conclusion

In summary, this work employed a rational coordination structure design strategy to select a system featuring a Co–N₆ coordination structure. Through precise modulation of the metal valence state, two MOFs with distinct metal valence states were successfully constructed. Using these MOFs as precursors, a pyrolysis process under a N₂ atmosphere was implemented, facilitating the transformation from the conventional Co–N₆ configuration into Co–N–C materials. This approach yielded final catalysts with similar overall architectures but distinct metal valence states, namely Co²⁺–N–C and Co³⁺–N–C. Electrochemical performance was systematically evaluated for Co–N–C materials with different metal valence states. The results demonstrate that, across various electrolyte solutions, the Co²⁺–N–C catalyst exhibits significantly enhanced catalytic activity for the ORR, OER, and HER compared to the Co³⁺–N–C catalyst. It is particularly noteworthy that in 0.1 M KOH electrolyte, the Co²⁺–N–C catalyst retained 91.3% of its initial electrocatalytic performance after 252 h of continuous operation at an applied potential of 1.616 V (vs. RHE). This design not only facilitates an in-depth analysis of the performance differences among various catalysts but also strongly confirms that catalysts with metal centers of different valence states exhibit significant disparities in their electrocatalytic performance. This work proposes an innovative research approach for exploring the intrinsic relationship between metal valence states and electrocatalytic performance, which is expected to provide valuable references for the further development of this field.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI): the content comprehensively covers electrochemical calculations, density functional theory (DFT) calculations, PXRD patterns, TGA curves, LSV curves, overpotential data, CV curves, Tafel slope calculations, proportional analysis tables of integrated areas, and comparison tables benchmarking the electrocatalytic performance of this study against previously reported catalysts of the same type. See DOI: https://doi.org/10.1039/d6ra00087h.

Acknowledgements

This research was supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LY24B010001, the National Natural Science Foundation of China under Grant No. 22478152, the Qin Shen Scholar Program of Jiaxing University (CD70623027).

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Footnote

† These authors contributed equally to this work.

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